The present application relates to nonaqueous electrolyte secondary cells and in particular to a nonaqueous electrolyte secondary cell having excellent cycle characteristics and capable of suppressing or preventing swelling during high-temperature storage. It also relates to a nonaqueous electrolyte secondary cell having leaktightness and excellent cycle characteristics.
Recent years have seen emergence of a variety of portable electronic apparatuses such as camcorders (videotape recorders with cameras), cellular phones, and portable computers, and size and weight reduction of these apparatuses have been achieved. To keep pace with such trends, development of cells as portable power supplies for these electronic apparatuses, in particular, secondary cells, is actively conducted. Of such batteries, lithium ion secondary cells have attracted much attention as a device that can achieve high energy density.
Further size and weight reduction of cells by using laminate films and the like as the exterior package of the cells to replace metal cans of aluminum, iron, or the like, is also underway.
For the secondary cells using metal cans as the exterior package, a suggestion has been made for improving the cycle characteristics or the like by controlling the amount of electrolyte solution and amount of voids per unit cell volume (refer to Japanese Patent Nos. 2646657 and 2757398 and Japanese Unexamined Patent Application Publication No. 2000-285959). These patent documents relate to a liquid-system cell using an electrolyte solution as the electrolyte and specify the amount of electrolyte solution to the voids to prevent leakage of the solution and prevent the inner pressure from increasing.
However, when the amount of electrolytic solution prescribed in these patent documents is applied to a secondary cell that uses a laminate film as the exterior package, the cell swells extensively during storage at high temperature, which is problem.
Swelling during storage at high temperature can be suppressed by reducing the amount of electrolytic solution in the cell. If the amount of the electrolytic solution is excessively reduced and the electrolytic solution does not completely fill the space around the active material, cell reaction rarely occur in portions inside the electrode not in contact with the electrolytic solution, and sufficient cell volume would not be achieved.
Moreover, as the charge and discharge operation is repeated and the electrolytic solution between the negative and positive electrodes is consumed, the discharge capacity of the cell gradually decreases before the negative and positive electrode active materials deteriorate. Moreover, there may occur problems of degradation in cycle characteristics and internal shorts attributable to deficiency of the electrolytic solution.
In order to increase the energy density, it is desirable to charge as much as active materials involving charge and discharge reaction as possible. This requires the electrolytic solution in an amount sufficient to allow lithium ions to travel between the positive and negative electrodes. If the amount of electrolytic solution is not sufficient and the electrolytic solution does not completely surround the active material, the part not in contact with the electrolytic solution does not react, and a sufficient cell volume is rarely obtained.
Moreover, as the charge and discharge operation is repeated and the electrolytic solution between the negative and positive electrodes is consumed, the discharge capacity of the cell gradually decreases before the negative and positive electrode active materials deteriorate. Moreover, there may occur problems of degradation in cycle characteristics and internal shorts attributable to deficiency of the electrolytic solution.
In order to overcome such a problem, for a cell using a metal cell can, there has been a suggestion for improving the cycle characteristics by controlling the volume of the nonaqueous electrolytic solution relative to the discharge capacity (e.g., refer to Japanese Unexamined Patent Application Publication No. 2-148576).
However, this problem is not sufficiently overcome for nonaqueous electrolyte secondary cells using laminate films described above.
That is, when a laminate film is damaged, the laminate film undergoes rupture more easily than a hard metal can, and leakage of solution may occur from the ruptured part. Thus, increasing the amount of electrolytic solution in aiming to enhance the cycle characteristics has led to a problem of easy leakage.